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Patented photographic lens system designed by Dennis Taylor

The Cooke triplet is a photographic lens designed and patented in 1893 by Dennis Taylor who was employed as chief engineer by T. Cooke & Sons of York. It was the first lens system that allowed elimination of most of the optical distortion or aberration at the outer edge of the image. The Cooke triplet is noted for being able to correct the five Seidel aberrations. The compound lens design consists of three air spaced simple lens elements: two biconvex (positive) lenses surrounding a biconcave (negative) lens in the middle.

According to Taylor, the lens design was derived by considering a cemented achromatic doublet consisting of one thin negative element and one thin positive element, both of equal power; such a doublet would result in a compound lens with zero net power but also a flat field of focus. However, by separating the elements, the resulting air gap would act as an additional lens element, giving the system a positive overall power while retaining the flat field characteristic. To correct the oblique aberrations, Taylor recommended splitting one element and mounting them on either side of the remaining element; although he patented versions in which either the negative or positive element was split, in his preferred embodiment, he split the positive element. A Cooke triplet comprises a negative flint glass element in the centre with a positive crown glass element on each side. In this design, the Petzval sum is zero, so the field of focus is flat. In other words, the negative lens can be as strong as the outer two combined, when one measures in dioptres, yet the lens will converge light, because the rays strike the middle element close to the optical axis. The curvature of field is determined by the sum of the dioptres, but the focal length is not.

Arthur Cox noted that anastigmat lenses were "almost exclusively, the logical development of two main types, the symmetrical lens, and the Cooke triplet of H. D. Taylor." One of the first derivatives was the Voigtländer Heliar, developed by Hans Harting in 1900 as a symmetrical modification of the original Cooke triplet. Although the contemporary Zeiss Tessar (1902) is said to be a development of the Cooke triplet, it is more accurately considered a parallel development descended from earlier, four element designs by Paul Rudolph, including the Protar and Unar. To increase the lens speed, either the rear element was further split into two, as patented by Edward Bausch in 1900 and H. W. Lee as the Speedic in 1924, or a positive meniscus element was inserted into the front air space, as in Ludwig Bertele's enduring Ernostar and Sonnar designs for Ernemann and Carl Zeiss AG, respectively.